1. The Field of the Invention
The invention generally relates to electrostatic discharge (ESD) protection for vertical cavity surface emitting lasers (VCSELs). More specifically, the invention relates to electrostatic discharge protection using integrated protective devices.
2. Description of the Related Art
Semiconductor devices have in recent times become nearly ubiquitous in their use. For example, semiconductor device are found in computer systems, televisions, radios, lighting control, amplifiers, etc. One particular area that semiconductors have found use is in light generating devices such as light emitting diodes (LEDs) and semiconductor lasers. One type of semiconductor laser is the vertical cavity surface emitting laser (VCSEL). VCSELs are formed on a semiconductor substrate by stacking a bottom distributed Bragg reflector (DBR) mirror on the substrate, an active region, including a pn diode junction, on the bottom DBR mirror and a top DBR mirror on the active region. Appropriate metal contacts are also formed on the VCSEL to provide contacts for applying appropriate bias and signal voltages.
One challenge that presents itself in semiconductor device use is the prevalence of electrostatic discharges (ESDs). ESDs result when static electricity builds up on one object and discharges into a second object at a different voltage potential. Static voltage potential differentials can be very high voltages, in the thousands of volts. ESDs through semiconductor junctions result in high peak current outputs that can damage or destroy the semiconductor devices. The rate of current rise can also cause current to pass through a semiconductor device in a normally non-conducting region, such as through oxide regions. This is due, in the oxide region case, to the capacitive nature of the oxide region and the alternating current like behavior of rising currents. Once current has passed through these non-conducting regions, the non-conducting regions may become at least partially conducting even for DC currents, creating a region where it becomes likely that the semiconductor will fail.
Several models exist for describing ESDs, including namely human, machine and charged device models. Each of these models is used to illustrate different impedances that may exist in the sources of an ESD. These three models are illustrated in FIGS. 1A, 1B, and 1C which illustrate the human, machine and charged devices models respectively.
Referring now to FIG. 1A, the human ESD model is illustrated. FIG. 1A illustrates a direct current (DC) power source 102. At a time t1, a switch 104 is closed to charge a capacitor 106. This models a static charge build-up. In actual practice, a charge may build up on an object when objects, especially when the objects are made of plastic or rubber materials, are rubbed against each other. When one object has more positively or negatively charged particles than another, there is a voltage between the two objects. With static charge build-up, this voltage may vary from zero to very high voltages in the thousands of volts or more. At a time t2, the switch 104 opens leaving a static charge on the capacitor 106.
In the human ESD model shown in FIG. 1A, various impedances are modeled. For example, FIG. 1A illustrates a resistor 108 which models the natural electrical resistance of the human body to current flow. In DC circuits, the resistor 108 provides a constant impedance to the DC current. A typical value for resistance of the human body is about 1500Ω. The human body model further includes an inductor 110. The inductor 110 represents inductances that exist in a discharge path such as through the leads of an object that is an electronic component, or through other electrical paths. Inductance impedes DC current flow according to an exponential relationship varying over time. As time increases from when a DC voltage is first applied to the inductor, the impedance from the inductor decreases. Illustratively, in the human model, at time t3, the charge built up on the capacitor 106 is discharged through a component 112 through the resistor 108 and the inductor 110 by closing a switch 114. The impedance of the resistor 108 and inductor 110 provides some protection to the component 112. However, depending on the type of component 112, the voltages and currents developed through the component 112 may cause irreparable harm.
A case more severe than the human discharge model shown in FIG. 1A is the machine discharge model shown in FIG. 1B. This model is similar to the human discharge model except that the resistor 108 of FIG. 1A is absent. This model represents an object with a charge build-up coming in direct contact with an object, such as a metal plane or other surface with a different charge. The charge build up discharges through the component 112 through the inductor 110 which represents inductances that exist in a discharge path such as through the leads of an electronic component or through other electrical paths. In the machine discharge model, the inductor 110 limits some of the severity of the discharge into the component 112.
Yet an even more severe case is the charged device discharge model illustrated in FIG. 1C. In this model, there is neither a resistor nor an inductor to protect the component 112. Thus lower voltage may cause severe damage to the component 112 when an electrostatic discharge discharges through the component 112.
Exposure to ESDs is one very common cause of VCSEL failure. Further, the smaller an aperture of emission in the VCSEL, the lower the amount of ESD voltage a VCSEL can withstand. As an example, a VCSEL with a aperture diameter of about 5 microns may be damaged when ESD levels are on the order of about 100V using the human ESD model. Smaller apertures are often used in single mode VCSELs. Thus single mode VCSELs may be more susceptible to ESDs than other types of VCSELs
To protect devices from electrostatic discharge, external components may be used to limit the amount of current through a component or to provide an alternate path for the discharge. However, using external components significantly increases the total size of packaging needed. This may be less desirable when there is a need or advantage to having smaller component packaging. Therefore, what would be new and useful is electrostatic discharge protection that can be implemented without significantly increasing the size of components.